Process for regenerating waste liquor for reuse in kraft pulping operation



Jan. 30, 1968 E. D. CANN 3,366,535

PROCESS FOR REGENERATING WASTE LIQUOR FOR REUSE IN KRAFT PULPINGOPERATION 2 Sheets-Sheet 1 Filed July 11, 1966 mm 9v aw EREQG r9,INVENTOR.

Everett Doug/as Conn BY wig 740M114 MILK/44,.

A TTOR/VEYS 3,366,535 EUSE E. D. CANN ERA Jan. 30, 1968 PROCESS FORREGEN TING WASTE LIQUOR FOR R IN KRAFT PULPING OPERATION Filed July 11,1966 2 Sheets-Sheet 2 Bug q INVENTOR.

5 a: m@ mnT m5 wuwmbkmy mmT .w@ wk mmmmsmm Y K H m5 Q5 E were Doug/asCalm ATTORNEYS United States Patent 3,356,535 PROCESS FOR REGENERATKNGWASTE LIQUQR FOR REUSE IN KRAFT PULPING UPERATHON Everett Douglas Canu,Freeport, liL, assignor of forty percent to William T. NeimanContinuation-impart of application Ser. No. 453,043,

May 4, 1965, which is a continuation-in-part of application Ser. No.279,917, Apr. 5, 1963. This application .luly 11, 1966, Ser. No. 564,303

4 Claims. (Cl. 162-30) ABSTRACT OF THE DESCLGSURE Cooking chemicals arerecovered from kraft pulping process waste liquors by: (1) adding tokraft black liquor a calcium compound (oxide, hydroxide, carbonate)sufiicient not only to provide an alkaline pH and a separable complexbut in sufficient excess to react with sodium carbonate formed onsubsequent heating, (2) heating the complex to burn off organics, toreduce sodium sulfate to the sulfide, and to calcine calcium carbonate,(3) dispersing the resultant smelt in water to form sodium hydroxide andinsoluble calcium carbonate, (4) separating insolubles, (5) cyclinginsolubles to step (1), and (6) recovering mother liquor as kraft whiteliquor.

This is a continuation-in-part of application Ser. No. 453,043, filedMay 4, 1965, now US. Patent No. 3,296,- 064, which in turn is acontinuation-in-part of application Ser. No. 270,917, filed Apr. 5,1963, and now abandoned.

This invention relates to the treatment of kraft black liquor, and moreparticularly concerns the recovery and reuse of cooking chemicals fromthe Waste or black liquor from a kraft cellulosic pulping operation. Theinvention provides a system whereby separate causticizing plantsheretofore required in conjunction with kraft black liquor processingare entirely eliminated.

The kraft process of converting natural cellulosic materials such aswood chips into pulp for the manufacture of paper and other cellulosicproducts essentially involves digestion of the chips with a cookingliquor containing sodium sulfide and sodium hydroxide. This liquor,termed white liquor, reacts with lignin, hemicelluloses, and otherconstituents of the wood to form a water soluble or dispersible mixturewhich, after removal of the cellulose pulp, is known as waste or blackliquor.

In virtually all kraft pulp processing plants it is essential that theblack liquor be processed for recovery of the active cooking chemicals.To this end, the black liquor is generally concentrated to about 45-70%total solids, and this concentrate is burned in a furnace or kiln toremove organic materials and to convert sodium sulfate (a processchemical make-up) to sodium sulfide by reduction with carbon. The moltenor semi-fluid chemical smelt withdrawn from the furnace or kiln isdissolved in an aqueous solution to give a characteristic green liquor,after which the green liquor is causticized by adding slaked lime(calcium oxide) prepared from calcined limestone (calcium carbonate). Inaqueous solution the slaked lime reacts with the sodium carbonate toform soluble sodium hydroxide and insoluble calcium carbonate. Thelatter is filtered off and then calcined in a lime kiln to recover thelime for reuse.

The resulting filtrate is the white liquor used in the kraft cookingprocess. It contains chiefly caustic soda and sodium sulfide, withsmaller amounts of sodium carbonate, sodium sulfate, sodium sulfite, andsodium thiosulfate.

' By reason of the fact that sodium compounds present in the smelt mayconsist of from 10 to about or more of sodium carbonate, it is evidentthat on some occasions a substantial amount of lime is required for thecausticizing plant to convert the sodium carbonate to sodium hydroxide.As a result, causticizing plants must have a large capacity, and arethus expensive in terms of both investment and operating costs.

Accordingly, an important object of the invention is to provide a kraftblack liquor recovery process which eliminates the need for separatecausticizing equipment.

A further object is to provide a process for recovering cookingchemicals from kraft black liquor whereby the need for concentrating theblack liquor before heating or burning is substantially reduced, or eveneliminated entirely.

Still another object is to simplify and reduce the investment andoperating costs of plants for recovering kraft white (or cooking) liquorfrom black liquor.

Other and further objects, aims, and advantages of the invention willbecome apparent from the ensuing description which is to be read inconjunction with the attached drawings wherein:

FIGURE 1 is a schematic flow sheet depicting one embodiment of theinvention;

FIG. 2 is an alternative embodiment which, when used in conjunction withthe system of FIG. 1, permits the separation of a complex composed ofcalcium oxide, hydroxide, or carbonate and components of the blackliquor;

FIG. 3 is an alternative embodiment utilizing acetone to facilitate theformation and separation of the separable complex; and

FIG. 4 is yet another alternative embodiment employing the acetonesystem of FIG. 3, and wherein provisions are made for recovering organicby-products from a pulp manufacturing operation.

Briefly, in accordance with the invention, it has now been discoveredthat expensive and elaborate causticizing equipment in kraft wasteliquor recovery processes may be eliminated entirely by adding to theblack liquor (or to a concentrated black liquor) before burning, calciumoxide, hydroxide, and/ or carbonate to provide, upon subsequent heatingin the same furnace with black liquor, sutficient calcium oxide (orhydroxide) to convert the sodium carbonate formed in the burningoperation to sodium hydroxide and calcium carbonate when the smelt isdissolved in an aqueous solution. The resultant calcium carbonate isseparated along with excess slaked lime from the solution and recycledto the process. Thus, an important feature of the invention is that thefurnace used to treat the black liquor serves simultaneously as acalcining furnace to reconvert the calcium carbonate to active calciumoxide.

An additional feature of the invention is the discovery that, undercertain conditions, the calcium oxide, hydroxide, or carbonate forms aseparable complex with the normally volatile components of the blackliquor. As a consequence, the complex may be separated from the blackliquor by physical means such as decanting, settling, centrifugation, orthe like, which thereby greatly reduces or even eliminates thepro-existing need of concentrating the black liquor by evaporation.

As mentioned previously, sodium sulfide and hydroxide are the activepulping chemicals in the kraft pulping process. The proportion of thesetwo chemicals is indicated by the term sulphidity, which simply meansthe relationship between them, and may vary from a fraction of onepercent to a fraction of a percent short of one hundred percent. Sincethe eausticization step only involves the sodium hydroxide, the amountof lime required in this step varies considerably. Furthermore, thisvariation is aggravated by the use of different amounts of chemicals inthe pulping operation. As a result, the actual amount of sodiumhydroxide used varies widely. In the case where the total chemicalcharge is low and sulphidity is high, very small amounts of sodiumhydroxide will be involved. In such a case, there may be insufiicientamounts of calcium compounds present to efiiciently form a separablecomplex. In order to make up this deficiency, magnesia may be used toserve this purpose. On the other hand, when an excessive amount ofcalcium compounds are required, as is the case when the total chemicalcharge is high and the sulphidity is low, it is advantageous to recyclesome of these calcium compounds not required to form the separablecomplex to the feed of the dryer or directly to one of the other twozones of the furnace. This technique relieves the excessive load on someof the equipment such as the mixer, reaction chamber, clarifier, filter,etc.

Still another feature of the invention is associated with the recentdiscovery that acetone effects the separation of the volatile componentsof the black liquor when used alone or when in the presence of calciumoxide, hydroxide, or carbonate by forming a separable complex. Thus, theformation of a separable complex by the action of the acetone and thenthe physical separation of this complex from the black liquor furtherreduces the need for evaporative concentration of the liquor. Moreover,the acetone complex permits ready separation of organic components ofthe black liquor for the production of commercial products.

It has been found that isopropyl alcohol, tertiary butyl alcohol, and toa lesser extent ethyl alcohol may be used in place of acetone, or may beused in combination with each other or with acetone to effect theseparation of some of the volatile components of the black liquor.Therefore, wherever the use of acetone is referred to, herein, it isunderstood that any one or combination of these alcohols or acetone maybe used. The conventional practice of oxidizing the volatile sulfurcompounds in the black liquor by treatment of oxygemcontaining gases toavoid losses of these compounds in the evaporation and burning stepsmaybe carried out in this process. However, except for embodiment No. 1,where there is no separation of a separable complex, it is desirable notto carry out this oxidation more than to oxidize the organic sulfurcompounds such as mercaptans and methyl disulfide, so as to avoid theoxidation of sodium sulfide to sodium thiosulfate o-r sulfate. Thepurpose of restricting this oxidation, when a separation is not made, isto avoid a build-up of sodium thiosulfate and sulfate in the liquorsystem. When the separable complex is removed from the liquor forpassage to the furnace, the sodium thiosulfate and sulfate present willnot pass to the furnace for conversion to the active pulping chemical,sodium sulfide.

Turning first to FIG. 1, the drawing depicts schematically a simplifiedsystem for recovering cooking chemicals from kraft black liquor. Theblack liquor in this case is obtained from the raft cooking of yellowpine chips, which are typically composed of about 50% cellulose fibers,about 30% lignin, about 16% other carbohydrates (e.g., hemicelluloses),about 3.3% resin and fats, and the remainder unidentified. Duringcooking, the lignin, carbohydrates, resin, and fats are dissolved by orreacted with cooking chemicals, or white liquor, to form the spent blackliquor. An illustrative black liquor will contain, after the cookingprocess, about 1220 weight percent of total solids, the remainder beingalmost entirely water, and will have an average density of about 11 Baumat 60 F.

In the process shown in FIG. 1, the kraft black liquor is obtained fromstorage 11 and is first conducted to absorber 12, which is used toremove noxious hydrogen sulfide, mercaptans, methyl disulfides, etc.from the due gases discharged from furnace 14. These gases, obtainedfrom the dryer 15 connected to the furnace 14, are conducted via line 16to a lower portion of the absorber 12, where they are contacted by adescending stream of kraft black liquor admitted near the top of theabsorber 12. Gases discharging from absorber 12 via conduit 18 are freefrom hydrogen sulfide and other annoying gases, and may be venteddirectly to the atmosphere via conduit 18. The addition of controlledamounts of air to line 16 will oxidize the sulfur containing compoundsin the black liquor as well as those in the flue gases.

Absorber 12 serves an additional function of reducing the pH of thenormally highly alkaline kraft black liquor to within the range of about7.5 to about 9.5, which has been found optimum in the event it isdesired to form a separable complex between the calcium oxide,hydroxide, or carbonate and normally volatile components of the blackliquor. Thus, depending on the concentration and amount of acidic gases,such as carbon dioxide, sulfur dioxide, hydrogen sulfide, etc. leavingfurnace 14 via conduit 16, a portion (or all) of the black liquor may beby-passed around the absorber 12 so as to control the pH within theforegoing optimum range.

However if, as in the illustrative embodiment of FIG. 1, there is noseparation of the calcium-containing complex, it is nonethelessdesirable to lower the pH to the range of about 7.5 to 9.0 merely toprevent having too alkaline a condition in any of the downstreamprocessing vessels. Furthermore, absorber 12 serves the additionalpurpose of recovering hydrogen sulfide and other sulfur containing gasesfrom the furnace 14, so as to reduce the amount of make-up sulfur orsulfur-containing compounds.

The stream of treated black liquor leaving absorber 12 via conduit 19 isthen conducted to a pair of multiple effect evaporators 2t 21, wherewater is removed under vacuum to concentrate the components of the blackliquor to about 40-75 weight percent total solids, optimally about 45-60weight percent total solids.

The strong black liquor leaving evaporator 21 via conduit 22 is thenconducted to a mixer 24, where it is commingled with a stream ofrecycled calcium carbonate and some calcium hydroxide supplied viaconduit 25. Recycle is added to mixer 2 1 via conduit 25, together withmake-up calcium carbonate, oxide, or hydroxide in an amount sufficientto provide, upon subsequent burning and heating of the mixture infurnace 14, sufiicient calcium oxide (or hydroxide) to convert theamount of the sodium carbonate formed in furnace 14 to sodium hydroxideand calcium carbonate in dissolver 39. For each mole of sodium carbonateobtained via the burning operation in furnace 14 there should,optimally, be one mole of calcium' oxide, hydroxide, or carbonate addedto the strong black liquor of conduit 32. Any excess calcium compound isnot undesirable as it is filtered out of the regenerated kraft liquorand recycled back via conduit 25. Also, any deficiency of calciumcompound results only in less effective regeneration. Additionally, itis generally found that calcination of calcium carbonate to calciumoxide in the furnace 14 proceeds almost quantitatively. Thus, forpreliminary orientation, it is assumed that such calcination occursquantitatively, with any adjustment in the minimum amount of calciummade thereafter to accommodate for insufficient calcination.

A stream of salt cake, i.e., sodium sulfate, is also added to mixer 24via conduit 26 to allow for processing losses in the cooking steps andfor any incidental sodium and sulfur losses in the waste liquor recoveryprocess. In furnace 14, sodium sulfate reacts with carbon to form carbondioxide and sodium sulfide, one of the active components of the kraftcooking liquor.

The stream leaving mixer 24 is conducted via valved conduit 28, valve 29(and conduits 3d, 31 in conjunction with the embodiment shown in FIG. 2)to the dryer 15. Dryer 15 desirably comprises an integral part of thefurnace 1d and is depicted basically in FIG. 1. Into dryer 15, thestream of heavy black liquor and recycled calcium compounds is conveyedand permitted to descend against an ascending stream of hot furnace fluegases. The final concentration at the bottom of dryer is approximately45-9O weight percent total solids, at which concentration the heavyblack liquor contains sufiicient combustible organic material to sustaincombustion when passed to the furnace 14.

As shown schematically in FIG. 1, the combination of dryer 15 andfurnace 14 is connected by flue gas conduit 32, and provides essentiallyfor a drying zone, a combustion zone and a reducing zone. The dryingzone, defined by dryer 15, is a vertically elongated chamber into whichheavy black liquor and recycled calcium compounds are conveyed viafeeder 34, and down through which the mixture descends against a risingstream of hot flue gases, admitted via conduit 32. This dryer may takewidely different forms to attain the same purpose. The gaseous efiluentof dryer 15 is discharged from the dryer via conduit 16 near the topthereof.

The burning or combustion zone and the reducing zone of the furnace 14are defined by the furnace 14 and, in the embodiment depicted in FIG. 1,by the upper and lower halves of the furnace 14. The upper half issupplied with combustion air via blower 35, and the lower or reducingzone supplied with controlled amounts of combustion air via blower 36.The black liquor and recycled calcium compounds discharged from thedryer 15 are conducted via a conduit 33 and fed into the upper oroxidizing zone of the furnace 14, where a stoichiometric excess of airsupplied via blower 35 causes complete combustion of the organicconstituents of the kraft black liquor, and thence, under reducingconditions, to effect the reduction of sodium sulfate to sodium sulfide.On some occasions it may prove advantageous to pass some, if not all, ofthe partially dried mixture in dryer 15 to the oxidizing Zone viaconduit 32.

Organic compounds in the black liquor are burned (or oxidized) in theoxidizing zone of furnace 14 and, as they descend in the furnace, arejoined by the air supplied via blower 36 which causes further combustionin the reducing zone of the furnace. The furnace flue gases are thenconducted via conduit 32 to the bottom of the dryer 15 countercurrent tothe process stream.

Conditions maintained in the lower or reducing zone of the furnace 14include a temperature sufficient to produce a semi-solid mixture orsmelt of the kraft cooking chemicals collecting at the bottom of thefurnace 14, and also to effect reduction of the sodium sulfate to sodiumsulfide and calcination of the calcium carbonate to calcium oxide. Thefurnace 14 temperatures in the oxidizing zone desirably fall within therange of about 1400-l900 F., optimally within the range of 1500-1800 F.,while in the reducing zone the temperatures are desirably within therange of about 1700 -2300 F., preferably within the range of aboutl800-2200 F. Under these conditions the desired chemical reactionsoccur, and the residue is in sufficiently semi-solid form to allowrecovery thereof following dispersion in dissolver 39.

Alternative dryer-furnace combinations may be employed herewith, and inthis respect attention is directed to the well-known CombustionEngineering Company dryerfurnace equipment and to the B-abcock andWilcox combination dryer and furnace, both of which are described inShreve Chemical Process Industries, pages 704-709 (McGraw-Hill, 1945).The furnace 14 may also be provided with steam generators for waste heatrecovery, so that the ultimate temperature of the flue gas released fromthe dryer 15 via stack 16 must be at a temperature above the dew pointto prevent condensation of the water present.

Additionally, it may be desirable under some circumstances to introducea reducing gas into the lower or reducing zone of furnace 14 to furtherenhance the reduction of sodium sulfate to sodium sufide. Reducing gasessuch as hydrogen sulfide, carbon monoxide, methane, or the like aresatisfactory for this purpose.

The chemicals which collect at the bottom of furnace 14 are, as beforestated, in the form of a semi-solid mass or smelt, and are dischargedvia conduit 38 to dissolver 39. Here the mass is dispersed in an aqueoussolvent, supplied vi-a conduit 40, to induce reaction between thecalcium oxide and the sodium carbonate to form sodium hydroxide andinsoluble calcium carbonate.

The solvent supplied via conduit 40 may be water or any convenientstream available at the plant which is predominantly water and is freeof substantial contaminants. Thus, this stream may comprise filtratefrom the pulp washers (i.e., dilute black liquor), in which eventeffective utilization and reuse of a substantial amount of the waternecessary for a pulp mill operation is achieved.

In any event, dissolver 39 produces a slurry containing the varioussalts and bases derived from the furnace 14 together with theirinter-reaction products. Before interreaction occurs, the salts arechiefly sodium carbonate and sodium sulfide in a usual ratio of betweenabout 2 4: 1, with lesser amounts of sodium sulfite, sodium thiosulfate,sodium sulfate, etc., while the bases are mainly calcium oxide (orhydroxide) with a minor amount of sodium hydroxide.

A substantial amount of inter-reaction occurs in the dissolver 39, theprincipal reaction being double displacement .between sodium carbonateand calcium hydroxide to form sodium hydroxide and insoluble calciumcarbonate. If desired, the dissolver 39 may be sufficiently large toprovide adequate residence time for this reaction to proceed tocompletion, but in the embodiment shown in FIG. 1 the dissolver 39 is ofrelatively small size, and a retention tank 41 connected to thedissolver 39 via conduit 42 is provided to afford sufiicient residencetime. The vapors evolved in dissolver 39 may be passed through acondenser (not shown) to remove water for reuse in the process, whilethe non-condensable gases, such as hydrogen sulfide, etc., may be passedto the furnace or a separate burner for destruction by burning and theresulting gases passed to line 16 for treatment.

The slurry in retention tank 41 is retained therein to permit the sodiumcarbonate-calcium hydroxide reaction to go substantially to completion.The resultant slurry is a mixture of cooking chemicals and insolublecalcium carbonate, together with any undissolved calcium hydroxide.Advantageously, the amount of water added via conduit 40 to thedissolver 39 was sufiicient to provide the proper cooking chemicalconcentration so that the solution, after removal of insolubles, issuitable for use as kraft cooking liquor.

The slurry taken from the bottom of retention tank 41 via conduit 44 isthen conducted to a continuous rotary filter 45, where undissolvedcalcium carbonate and any calcium hydroxide are separated as apredominately solids stream from the filtrate. The solids stream is thusavailable for recycle via conduit 25 to the mixer 24, while the filtrateis withdrawn via conduit 46 as kraft white liquor.

It is therefore apparent that the system of the invention provides asimple yet advantageous process for recovering cooking chemicals fromkraft black liquor. It is particularly notable that, in contrast toconventional black liquor recovery processes, there is no need for aseparate causticizing plant. Thus, this element of plant constructionand maintenance cost is avoided entirely.

Another embodiment of the invention is depicted schematically in FIG. 2,which is to be taken in conjunction with the system of FIG. 1. Thesystem of FIG. 2 provides further economies in a kraft black liquorrecovery process in that it reduces the necessary amount of evaporationcarried out by evaporators 20, 21 (FIG. 1) and, under appropriateconditions, may even eliminate the need for these evaporators.

The embodiment of FIG. 2 is based largely on the recognition that undercertain circumstances a separable assasss complex is formed between mostof the normally volatile components of the black liquor and calciumoxide, hydroxide, or carbonate. Thus, by separating this complex fromthe black liquor by such physical separation techniques as settling,clarifying, filtration, centrifugation, or the use of hydrocyclones, thevaluabie constituents of kraft black liquor may be recovered and some ofthe volatile components are destroyed by burning without the need forsubstantial evaporation of the water. Furthermore, unused pulpingchemicals in the black liquor are recovered without having to beconverted from the inactive pulping form, sodium carbonate, to sodiumhydroxide by passing around the furnace in the mother liquor.

Conditions favorable to the formation of such a separable complex dependlargely on the particular composition of kraft black liquor, which inturn depends on the composition of the wood chips and on the conditionsexisting in the wood chip digestors. It has been found, however, that asubstantial excess of calcium compound, and a pH in the lower alkalinerange, are generally conducive to separable complex formation. Thus, apH above about 8.5 and up to about 10.0, and a calcium compoundconcentration of between about 10-80 weight percent of total normallynon-volatile solids, will usually result in the formation of a separablesolids complex.

When employing the embodiment of FIG. 2, the absorber 12 and theevaporators 20, 21 of FIG. 1 may be reduced in size or else eliminatedentirely. In addition, valve 29 of FIG. 1 is closed to direct the streamleaving mixer 24 (FIG. 1) into conduit 30, indicated as A in both FIGS.1 and 2. The returning stream from the system of FIG. 2 re-enters of thesystem of FIG. 1 via the conduit 31, shown as B.

Turning to FIG. 2, the mixture of black liquor and recycled calciumcompounds is conducted as an aqueous slurry via conduit 39 to heatedreaction chamber 48, Where additional reaction is permitted to form thecomplex between calcium compounds and the normally volatile componentsof the black liquor. The consequent slurry is discharged from the bottomof the reaction chamber 48 and conducted via conduit 49 to a clarifier59. After leaving reaction chamber 48, the stream may pass through thefollowing types of equipment (not shown) to promote the separation ofthe separable complex; vacuum evaporator to cool and concentrate thestream and retention chests to provide adequate dwell (residence) time.

In the clarifier t} a clarified aqueous solution is withdrawn near thetop via conduit 51, while the thickened complex is withdrawn near thebottom via conduit 52. The stream leaving via conduit 51 contains sodiumhydroxide and sulfide which may be recovered directly, while the streamleaving via conduit 52 contains most of the organic components and aminor proportion of the inorganic components of the black liquor. Thethickened complex-containing stream of conduit 52 is conducted to acontinuous filter 54, which removes additional Water and provides arelatively dry complex. The additional water is discharged via conduit55, which may be combined with the stream of conduit 51, While the mainstream leaves filter 54 via conduit 31. From the conduit 31 it isreturned to the system of FIG. 1, which operates as describedpreviously. In this embodiment the make-up salt cake is added to line 34or directly to the oxidizing or reducing zones of the furnace.

FIG. 3 depicts schematically an alternative embodiment of the inventionwhich incorporates discoveries recently made by others to the effectthat the addition of acetone causes the formation of a separable complexfrom the volatile components of the kraft black liquor. Such a complexis independent of the presence of calcium con pounds, but in theembodiment of FIGURE 3, a separable double complex of the calciumcompound-volatile component complex and the acetone-volatile componentcomplex is employed.

The numbers on components in FIG. 3 have been assigned to facilitateeasy reference to, and comparison with, corresponding elements inFIG. 1. An element or component in FIG. 3 that corresponds substantiallyto a similar elementof FIG. 1 has been assigned a number that is onehundred more than the element of FIG. 1. Thus, for example, evaporator21) of FIG. 1 has been designated as evaporator 12d of FIG. 3. Numbersin excess of 156 are Without corresponding elements in FIG. 1.

Turning to FIG. 3, a kraft black liquor admitted from source 111 mayfirst be conducted to an absorber, not shown, corresponding to absorber12 in FIG. 1, and thence to a double effect evaporator 121B, 121. Theseevaporators may be eliminated, but according to optimal practice areoperated to provide a discharge stream via conduit 122 that is at leastabout 20% by weight of total solids.

The stream leaving the bottom of the evaporator 121 via the conduit 122is conducted to an upper portion of acetone absorber 156, where itabsorbs uncondensed acetone vapor admitted to the bottom of the absorber156 via conduit 158; conduit 15% contains both vaporized and condensedacetone, as well as water and other components. An acetone-free streamis vented from acetone absorber via conduit 159.

An alternative procedure to this phase of the process involves the useof a fraction-action column in line 158, so that the water present maybe removed for reuse in the process and the acetone vapors alone arepassed directly to acetone absorber 156.

In the acetone absorber 156 formation of a complex of the volatilecomponents of the black liquor is initiated by the use of acetone. Theextent of such complex formation will depend, of course, on the relativeamount of acetone to volatile components and on the amount of waterpresent, and also on the residence time in the obserber 155. At least a1:1 ratio of acetone and black liquor is required to develop thiscomplex with acetone alone.

The stream leaving the bottom of acetone absorber 156 conducted viavalved line 16%, provided with valve 161, to mixer 124, where it iscommingled with a recycled calcium carbonate and by roxide streamsupplied via conduit 125. As in the case of the'systcm of FIG. 1, theamount of calcium supplied via conduit 125 is sufficient to provide,after heating in the furnace 114, sufiicient calcium oxide or hydroxideto convert a substantial amount, e.g., preferably more than half, of thesodium carbonate formed in the furnace 114 to sodium hydroxide (andcalcium carbonate). It is noted that, in the embodiment of FIG. 3,make-up salt cake is not added to the mixer 24, but to line 172 ordirectly to the oxidizing or reducing zones of the furnace.

The slurry leaving mixer 124 via conduit128 is conducted to a clarifier162, which separates the complex, as a bottoms product discharged viaconduit 164, from the mother liquor released via conduit at the top ofthe clarifier 162. Alternatively, mixer 124 may advantageously precedeacetone absorber 156.

The mother liquor of conduit 165 contains chiefly water and acetone,together with any dissolved salts, and is stripped of acetone in vacuumstill 166, a plate-equipped stripping column provided with internalreflex. Acetone leaves vacuum still 166 as a vapor via conduit 168,where it proceeds via conduit 15% and a cooler 159 to the acetoneabsorber 156, described previously. The bottoms from vacuum still 166 ischiefly Water containing dissolved salts, and is conducted via conduit169 to the dissolver 13$, where it constitutes all or a portion of thewater used to form green liquor with the smelt 133 discharging from thefurnace 114. in cases where bottoms in conduit 169 contain solids, thesemay be filtered by using a filter precoated with some of the recycledcycled calcium carbonate and hydroxide. The solids thus separated arepassed todryer 115.

Returning to clarifier 162, the complex released from 75 the clarifier162 via conduit 164 is conducted to a separate vacuum still 170,designed and operating similarly to that of vacuum still 166, where anacetone and water vapors stream is released via top conduit 171 and isconducted to conduit 158, while a bottoms acetone-free stream isdischarged via conduit 172 to the dryer 115.

The dryer 115-furnace 114 of FIG. 3 operates similarly to that of dryer15-furnace 14 of FIG. 1, with one exception. It is preferred, in theembodiment of FIG. 3, that flue gases discharging from the dryer 115 viaconduit 116 be stripped of their hydrogen sulfide and/ or sulfur dioxidecontent in an absorber 173 supplied with a soda ash or caustic sodasolution via conduit 175. Absorber 173 is thus an effective alternate ofabsorber 12 of FIG. 1, although the arrangements are, in effect,interchangeable. The stream leaving the bottom of absorber 173, viaconduit 176, is combined with the stream in conduit 172 containing heavyblack liquor, and sent to the dryer 115. The use of absorber 173 avoidsdecreasing the alkalinity of the incoming black liquors.

Progress through the dryer 115, furnace 114, dissolver 139, retentiontank 141, and filter 145 correspond to the operation of the equivalentelements in the system of FIG. 1.

It is evident, therefore, that the embodiment of FIG. 3 permits ofsubstantial economies in evaporating and drying costs for the reasonthat these operations are applied to a separable complex rather than toan entire stream of volatile components dissolved or dispersed in alarge amount of aqueous medium. Since evaporating and drying arenormally expensive operations, the system of the invention not onlyeffects economies by saving on these, but contributes to the net streamproduction of the waste liquor recovery process.

Turning now to FIG. 4, a further embodiment is illustrated which is usedin conjunction with the embodiment of FIG. 3. In the FIG. 4 embodimentprovisions are made for recovering organic by-products from a kraftpulping operation by processing the volatile components of the blackliquor.

To conduct the system of FIG. 4, the valve 161 of FIG. 3 is closed andcorresponding valves leading to conduits 178 or C, and from conduits 179or D and 180 or E are opened.

In the system or embodiment of FIG. 4 a slurry or suspension ofacetone-separated volatile components of the black liquor dispersed inthe remaining black liquor is treated for the recovery of organicby-products. This is accomplished merely by concentrating the complex inclarifier 181, and transmitting the separated volatile components as ablack viscous liquid via conduit 182 to a zone 184 for the recovery oforganic by-products. After such recovery, the residue is returned to thesystem of FIG. 3 via conduit 180 or E, while the liquid fraction ofmaterial from conduit 178 is returned via conduit 179 of D, andprocessed in the normal manner. Make-up acetone is preferably added tothe acetone absorber 156 or may be added at any point in the streamprior to vacuum stills 166 and 170.

The optimum pH for the formation of the volatile component complex bythe action of acetone has been found to be about 13.5. As mentionedpreviously the incoming black liquor is not treated with the gases fromdryer 115 to avoid lowering the pH. The optimum pH of 13.5 mayadvantageously be attained by adding sodium hydroxide for the sodiummake-up, in which case the required sulfur make-up can be obtained byadding elemental sulfur to the pulping or white liquor prior to thepulping operation or directly to the pulp digesters. This addition ofsodium and sulfur separately may be used to supplement or replacemake-up salt cake.

Various modifications to the foregoing four embodiments will be evidentfrom the above description. For example, all or a portion of the furnacefiue gas may be sent to an absorber for contact with the original blackliquor, or the absorber may be omitted entirely. Similarly, the

10 black liquor may be concentrated by evaporation, or again this may beomitted. Whether such units are in fact employed depends largely oneconomic aspects, that is, a comparison of the costs of theirinstallation and operation as opposed to the savings obtained by theiruse.

While the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives,modifications, and variations will be suggested to those skilled in theart in light of the foregoing description. Accordingly, it is intendedto embrace all such alternatives, modifications, and variations as fallWithin the spirit and broad scope of the appended claims.

I claim as my invention:

1. A process for recovering cooking chemicals from kraft black liquor,which black liquor contains the reaction products from cookingcellulosic materials with aqueous caustic and sodium sulfide,comprising:

(1) adding to said black liquor a material selected from the groupconsisting of calcium oxide, hydroxide, and carbonate in an amount toprovide (a) a pH above about 8.5, (b) a calcium compound concentrationof between about 10-80 weight percent of normally non-volatile solids,and (c) after the heating step below, sufiicient calcium oxide orhydroxide to thereafter convert a substantial amount of the sodiumcarbonate formed in said heating step to sodium hydroxide and calciumcarbonate,

(2) (a) heating said material and said reaction products in an oxidizingzone at a temperature within the range of about 1400-1900 F., underconditions effective to oxidize organic materials, and (b) furtherheating said material and said reaction products with a stoichiometricdeficiency of air in a reducing zone at a temperature within the rangeof about 17002300 F., under conditions effective to (1') cause furthercombustion of organic materials, (ii) reduce sodium sulfate to sodiumsulfide and (iii) calcine calcium carbonate to calcium oxide, andthereby form a smelt comprising chiefly sodium sulfide, calcium oxide,and sodium carbonate,

(3) dispersing said smelt in an aqueous solvent to convert said calciumoxide and said sodium carbonate to sodium hydroxide and insolublecalcium carbonate,

(4) separating insoluble calcium carbonate and any excess insolublecalcium hydroxide from the mother liquor,

(5) cycling said separated calcium carbonate and any excess insolublecalcium hydroxide to step number (1) above, and

(6) recovering mother liquor as kraft white liquor.

2. A process for recovering cooking chemicals from kraft black liquor,which black liquor contains the reaction products from cookingcellulosic material with aqueous caustic and sodium sulfide, comprising:

(1) adding to said black liquor a material selected from the groupconsisting of calcium oxide, hydroxide, and carbonate in an amount toprovide (a) a pH above about 8.5, (b) a calcium compound concentrationof between about 10-80 weight percent of normally non-volatile solids,and (c) after the heating step below, sufiicient calcium oxide orhydroxide to thereafter convert a substantial amount of the sodiumcarbonate formed in said heating step to sodium hydroxide and calciumcarbonate,

(2) heating said material and said reaction products under conditionseffective to (a) oxidize organic materials under oxidizing conditions,(b) reduce sodium sulfate to sodium sulfide under reducing conditions,and (c) calcine calcium carbonate to calcium oxide, to thereby form asmelt comprising chiefly sodium sulfide, calcium oxide, and sodiumcarbonate,

(3) dispersing said smelt in an aqueous solvent to convert said calciumoxide and said sodium carbonate to sodium hydroxide and insolublecalcium carbonate,

1 l l 2 (4) separating insoluble calcium carbonate and any acetone fromsaid separated complex, and subjecting the excess insoluble calciumhydroxide from the mother residue of said complex to said heating stepnumber (2). liquor, (5) cycling said separated calcium carbonate and anyReferences l d excess insoluble calcium hydroxide to step number 5UNITED STATES PATENTS and 1,638,061 8/1927 Rinman l62 30 (6) recoveringmother liquor as kraft White liquor. I 1,743,080 1/1930 Bradley et aL162 16 3. Process of clann 2 including the step of separatmg 2,034,8333/1936 Rinman 162 30 a separablev complex from the remaining blackliquor and 2 774 12 1 5 Barton et aL the step of subjecting said complexto said heating step 10 number FOREIGN PATENTS 4. Process of claim 2including the step of mixing ace- 253,035 9/ Great l ln. tone With saidmaterial and said black liquor to form a 91,420 4/1938 Norwayseparablecomplex, separating said separable complex from the remaining blackliquor, distillatively removing 15 LEON BASHORE Pl'mmry Exammer'

